There are various transmission channels used for transmitting data or information. Telephone lines consisting of copper wires were used for well over a hundred years for transmitting both voice and data. Radio transmission of radio signals have been around for almost a hundred years. A radio station sends a radio signal out over the airwaves to be received by a radio set. As is known, a radio station has programming which may include music, news, or programs. Satellites are an example of another transmission channel in which a satellite dish positioned a first location is used to transmit a signal to a satellite to be beamed or sent from the satellite to a second satellite dish positioned at a location remote from the first location. More recently cellular communication systems have been used to communicate between cell phones. An enormous amount of data is being sent using cellular communication systems. At this point in time it is essential to be able to increase the data throughput over any transmission channel that is used. It is also important to address the problem of signal degradation during transmission of the signal. Some problems encountered when transmitting a signal over a transmission channel include transmission path delay, interference, and non-linearity.
Some transmission techniques or schemes that have been developed and used in an effort to increase data throughput over a transmission channel are Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation, QAM (Quadrature Amplitude Modulation), QPSK (Quadrature Phase Shift Keying), PSK (Phase Shift Keying), and APSK (Amplitude and Phase Shift Keying).
Amplitude Modulation is a modulation technique used for transmitting information by use of a radio carrier wave. A sinusoidal carrier wave has its amplitude modulated by an audio waveform before transmission. The audio waveform modifies the amplitude of the sinusoidal carrier wave. Some disadvantages associated with the use of an amplitude modulation signal are that an amplitude modulation signal is not efficient in terms of its power usage, it is not efficient in terms of its use of bandwidth, it requires a bandwidth equal to twice that of the highest audio frequency, and it is prone to high levels of noise.
Frequency Modulation is a modulation technique that encodes information in a carrier wave by varying the frequency of the wave. Although Frequency Modulation has some advantages over Amplitude Modulation some disadvantages include that it requires a more complicated demodulator and that is has a poorer spectral efficiency than some other modulation techniques.
QAM is a form of multilevel amplitude and phase modulation that modulates a source signal into an output waveform with varying amplitude and phase. A system that employs QAM modulates a source signal into an output waveform with varying amplitude and phase. A message to be transmitted is mapped to a two-dimensional four quadrant signal space or constellation having signal points or phasors each representing a possible transmission level. Each signal point in the constellation is referred to as a symbol. The QAM constellation has a coordinate system defined by an I or in-phase axis and a Q or quadrature axis or an IQ plane. A symbol may be represented by both I and Q components. One of the disadvantages of the use of QAM is that for the higher data rates the peak to average power ratio is high. For example, in a typical constellation diagram for 16 QAM, it can be seen that there are four possible power levels. As the order of the modulation increases, so the number of power levels needed increases. All of this results in ever higher peak to average power ratios being experienced.
QPSK has a synchronous data stream modulated onto a carrier frequency before being over a channel. The carrier can have four states such as 45°, 135°, 225°, or 315°. QPSK also employs a quadrature modulation where the signal points can be described using two orthogonal coordinate axes, such as the IQ plane. With conventional QPSK, there is the problem that the transition between two diagonal transmission symbol points in the complex plane passes through the zero point. In the transition between these diagonal transmission symbols, a lowering of the amplitude may occur, the so-called envelope, to practically zero. On the receiver side, it complicates the necessary synchronization and favors nonlinearities in the transmission path, signal distortion, and unwanted intermodulation.
PSK is another digital modulation process which transmits a message by modulating the phase of a carrier wave. One disadvantage of using PSK is that when a high order PSK constellation is used the error-rate becomes too high.
As the name APSK indicates, this form of modulation uses amplitude and phase shift keying. In this modulation scheme a signal is conveyed by modulating both the amplitude and the phase of a carrier wave. Amplitude and frequency shift keying is able to reduce the number of power levels required to transmit information for any given modulation order.